A warming device that may be used perioperatively includes a clinical garment and convective apparatus with multiple sections supported on the inside of the garment. The sections are for receiving and distributing warmed, pressurized air and then circulating the distributed air through permeable surface areas. The permeable surface areas include areas of different permeability.
Convective devices that transfer heat to a human body are known. For example, there are devices that receive a stream of warmed pressurized air, inflate in response to the pressurized air, distribute it within a pneumatic structure, and emit the warmed air onto a body. These devices are typically called “convective thermal blankets” or “covers”. Arizant Healthcare Inc., the assignee of this application, makes and sells such thermal blankets under the BAIR HUGGER® brand. One such device is the Model 522 Upper Body Blanket.
Use of the term “convective” to denote the transfer of heat between a warming device and a body refers to the principal mode of heat transfer, it being understood that heat may at the same time be transferred between a convective warming device and a body by conduction and radiation, although not to the degree of convection.
Convective warming has been used with increasing frequency to prevent or mitigate hypothermia during medical treatment. However, with the expansion of convective warming to treat core body cooling, it becomes apparent that it has manifold medical uses.
For example, a recent invention disclosed in the referenced PCT application adapts a clinical garment such as a robe or gown to receive a convective warming device in order to warm a person wearing the garment in a clinical setting for comfort and mobility of the person. Arizant Healthcare Inc., the assignee of this application, makes and sells such warming devices under the BAIR PAWS® brand. These devices are intended to warm patients prior to surgery, and there is a need to further adapt such a combination for use perioperatively.
The term “perioperative” is defined in the PDR Medical Dictionary, Second Edition, (Medical Economics Company, 2000), as “around the time of operation.” The perioperative period is characterized by a sequence including the time preceding an operation when a patient is being prepared for surgery (“the preoperative period”), followed by the time spent in surgery (“the intraoperative period”), and by the time following an operation when the patient is closely monitored for complications while recovering from the effects of anesthesia (“the postoperative period”).
According to Mahoney et al. (Maintaining intraoperative normothermia: A meta-analysis of outcomes with costs. AANA Journal. 4/99;67,2:155–164.), therapeutic warming is employed during at least the intraoperative period in order to prevent or mitigate a constellation of effects that result from hypothermia. In fact, it is increasingly manifest that maintenance of normothermia perioperatively enhances the prospects for a quick, successful recovery from surgery. The effectiveness of therapeutic warming depends upon delivery of enough heat to a patient's body to raise the patient's core body temperature to, or maintain it within, a narrow range, typically near 37° C. This range is called “normothermic” and a body with a core temperature in this range is at “normothermia.” Hypothermia occurs when the core body temperature falls below 36° C.; mild hypothermia occurs when core body temperature is in the range of 34° C. to 36° C. Therefore, “perioperative therapeutic warming” is warming therapy capable of being delivered during one or more of the perioperative periods for the prevention or treatment of hypothermia.
Therapeutic warming is contrasted with “comfort warming” which is intended to maintain or enhance a patient's sense of “thermal comfort”. Of course, therapeutic warming may also comfort a patient by alleviating shivering or a feeling of being cold, but this is a secondary or ancillary effect. And conversely, there may be a secondary or ancillary therapeutic effect derived from the application of thermal comfort treatments such as the relief of patient anxiety. Thermal comfort is a subjective notion; however, the environmental conditions necessary to produce a sense of thermal comfort in a population of human beings are known and well tabulated. For example, Fanger (Thermal Comfort: Analysis and Applications of Environmental Engineering. Danish Technical press, Copenhagen, 1970) defines thermal comfort as “that condition of mind which expresses satisfaction with the thermal environment.” Even when a patient is normothermic, less than ideal environmental conditions can result in acute feelings of discomfort. Under normothermic conditions, thermal comfort is largely determined with reference to skin temperature, not core body temperature. Comfort warming is warming applied to a patient to alleviate the patient's sense of thermal discomfort.
Therapeutic warming may be indicated during any one or more of the perioperative periods. For example, for a short operation in a surgery with no warming equipment available, a person may be warmed preoperatively in a preparation area to raise mean body temperature to a level higher than normal in order to store enough thermal energy to maintain normothermia, without heating, intraoperatively. After surgery, it may be necessary to apply therapeutic warming in a recovery area to raise the core temperature to normothermia and maintain it there for a period of time while anesthesia wears off. Alternatively, for a long surgery in an arena with heating equipment available, a person may be warmed for comfort before surgery and warmed therapeutically during and after surgery.
Thermal blankets are typically used for therapeutic heating. An example is found in U.S. Pat. No. 6,524,332, “System and Method for Warming a Person to Prevent or Treat Hypothermia”, commonly owned with this application. Thermal blanket designs have converged on a lightweight inflatable structure made of a flexible material which distributes warmed pressurized air over or against a permeable blanket surface that faces some portion of a patient's body and emits the distributed heated air through small apertures or interstices in the surface.
Thermal blanket design has been adapted for comfort warming by convective means such as those described in the referenced U.S. patent applications, and the referenced Publication No. WO 03/086500.
When delivered by convective devices, therapeutic warming is distinguished from comfort warming by intended effects and by the parameters of heat delivery that produce those effects. In this regard, a convective warming system typically includes a source of warmed pressurized air (also called a heater/blower unit, a forced air warming unit, a heater unit, etc.), a convective device such as a thermal blanket (which is, typically, inflatable), and a flexible conduit or air hose connecting the heater/blower unit with the thermal blanket. Use of such a system for a particular type of warming requires delivery of warmed air through a convective device at parametric values that achieve a particular objective. The conditions by which a convective device produces thermal comfort in normothermic individuals at steady state are significantly different from those necessary to treat hypothermia. Typically the conditions for thermal comfort are met in a system with a relatively low capacity heater/blower unit, while those in a therapeutic warming system are achieved with a relatively high capacity heater/blower unit. The different capacities have led to use of air hoses with different capacities, with those delivering air flow for thermal comfort typically having smaller diameters than those serving a therapeutic warming requirement. The result is a divergence of designs leading to installation of different air delivery infrastructures for therapeutic and comfort warming.
Health care cost is an issue of national importance. The cost of warming perioperatively by convection is directly related to the number of perioperative periods in which a person is warmed; the cost increases when different convective warming apparatus are used in different periods to accomplish different goals. For example, when comfort and mobility are objectives of warming a person during the preoperative period and therapy is the objective of warming during one or more of the intraoperative and postoperative periods, it is presently necessary to use different convective warming configurations. Manifestly, if one convectively-operating warming device could be used or adapted to be used perioperatively, significant savings in thermal care could be realized.